You can get your factor-of-1,000 from a combination of:
Valuing instrumental effects more than short term suffering (perhaps because of taking a longtermist lens in which case this could get you more than a factor-of-1,000 by itself; but one doesn’t need to adopt longtermism to think some more moderate factor-of-adjustment is correct here);
Upweighting the climate costs to count economic impacts on the poorest more than economic impacts on richer people (probably correct and important, but I’m not sure if this is like a factor-of-2 adjustment or a factor-of-20 adjustment);
Upweighting climate costs to account for tail risks in addition to central projections of economic cost (if 90% of your worry about climate change is about tail risks this should be a factor-of-10 adjustment; different people will have different takes on that);
The direct moral importance of suffering of different beings (reasonable views vary a lot, but Jeff’s linked post seems like a careful thinker trying to have a reasonable take and arriving at figures in the vicinity of factor-of-1,000 just from this factor)
I don’t think it’s that hard for combinations of these factors to push you over to “climate effects matter more”.
You can get your factor-of-1,000 from a combination of:
Valuing instrumental effects more than short term suffering (perhaps because of taking a longtermist lens in which case this could get you more than a factor-of-1,000 by itself; but one doesn’t need to adopt longtermism to think some more moderate factor-of-adjustment is correct here);
Upweighting the climate costs to count economic impacts on the poorest more than economic impacts on richer people (probably correct and important, but I’m not sure if this is like a factor-of-2 adjustment or a factor-of-20 adjustment);
Upweighting climate costs to account for tail risks in addition to central projections of economic cost (if 90% of your worry about climate change is about tail risks this should be a factor-of-10 adjustment; different people will have different takes on that);
The direct moral importance of suffering of different beings (reasonable views vary a lot, but Jeff’s linked post seems like a careful thinker trying to have a reasonable take and arriving at figures in the vicinity of factor-of-1,000 just from this factor)
I don’t think it’s that hard for combinations of these factors to push you over to “climate effects matter more”.